With transmission methods using copper lines, there is the problem of transferring exchange-end transmission/reception devices and subscriber-end transmission/reception devices connected via at least one data transmission path from a deactivated state or from a standby state to an activated state, this operation being referred to below as a “warm start”.
xDSL technologies involve various types of use/carrier methods for line-conducted data stream transmission from an exchange to a subscriber end.
By way of example, the following DSL (DSL=Digital Subscriber Line) methods are used:                (i) ADSL: asymmetric DSL,        (ii) VDSL: Very High Data Rate DSL,        (iii) SDSL: Symmetrical Single Pair DSL (DSL with a symmetrical single pair line), and        (iv) S(H)DSL: (SDSL at a high data rate).        
These transmission methods using the at least one data transmission path are based on exchange-end transmission/reception devices and subscriber-end transmission/reception devices, the problem being that both transmission/reception devices need to be transferred to an energy saving mode, or need to be deactivated, for energy saving reasons.
To change from an energy saving mode or a standby mode to an operating mode, the deactivated transmission/reception devices need to be reactivated using a warm start sequence.
In this context, it can be assumed that a first warm start sequence has been preceded by a “cold start”, where data transmission has been activated on a line.
The exchange-end transmission/reception devices and the subscriber-end transmission/reception devices adapt themselves in this case to the parameters or properties of the line in the data transmission path.
Disadvantageously, this operation typically takes at least 30 seconds, which means that a warm start, which comprises a warm start sequence and requires much shorter time periods for activation, is absolutely essential. In this context, all subsequent activation operations require that knowledge of the properties of the line in the data transmission path be put into practice in a fraction of the aforementioned time (30 seconds).
All of these activation operations are referred to as “warm starts”. Internationally standardized warm start sequences exist for ISDN-U interfaces, for example, as described in the literature reference:
“ETSI TS 102 080, Integrated Services Digital Network (ISDN) basic rate access, Third Edition, November 1998” and in the literature reference:
“ANSI T1.601-1988, Integrated Services Digital Network (ISDN), ANSI, September 1988”.
For xDSL, particularly for S(H)DSL methods, various methods are known which all have serious drawbacks for existing transmission/reception devices. Such methods are described in the literature references
“Infineon, “SDSL Warm Start Capability”, ETSI-Meeting TM6 Amsterdam, December 1999, 994t51a0”;
“Adtran, “Warm Start Considerations”, ETSI-Meeting TM6 Amsterdam, December 1999, 994t59a0”; “Infineon, Adtran, “SDSL Warm Start”, Helsinki, May 2000, 002t25a1”;
“Infineon “Warmstart Wake-up tones” ETSI-Meeting TM6 Vienna September 2000, 003t42a0”;
“Conexant Systems “Warm start wake up signals” ETSI-Meeting TM6 Monterey, November 2000, 00t24a0”;
“Conexant Systems “M-Sequence based line probe signals for both keep-alive and warmstart wake-up signals” ETSI-Meeting TM6 Sophia Antipolis, February 2001, 011t25a0”; and
“Infineon, “warmstart-Sequence”, ETSI-Meeting TM6 Ghent, May 2001”; “Infineon “Warmstart wake-up tones”, Ghent, May 2001”.
FIG. 3 shows a flowchart in block form to illustrate a method for transmitting data streams in accordance with the prior art. Following a start step S100, subscriber-end activation is effected in a subscriber-end activation step S101. The subscriber-end activation provokes exchange-end activation, which is performed in an exchange-end activation step. This is conventionally followed by subscriber-end synchronization (step S107) and finally exchange-end synchronization (step S108): after steps S101-S108, a data stream can be transmitted in a data stream transmission step S111. Following data stream transmission, the procedure is stopped (step S112).
A main drawback of conventional methods for transmitting data streams where transmission/reception devices can be deactivated to a standby state is that, before synchronization or before a specific warm start, a known far end signal needs to be provided in order to detect a change in the far end signal as a consequence of a change in properties of a line in the at least one data transmission path since activation last occurred.
By way of example, interference such as temperature fluctuations or changes in air humidity has a disadvantageous effect on line properties and can alter the line properties in an unforeseeable manner.
Another drawback of known methods is that transmission/reception devices are available which require a time period without a far end signal before synchronization, in order to adapt themselves to a change in an echo in their own transmitted signal, which is also caused, inter alia, by a change in line properties in the at least one data transmission path.
Disadvantageously, a fixed warm start sequence in the exchange-end transmission/reception devices and in the subscriber-end transmission/reception devices would mean a high level of complexity and would lengthen a warm start sequence unnecessarily, resulting not only in an increase in costs but also in a reduction in the specific advantages of a warm start.